The invention relates to a method of shortening the braking distance in critical driving situations and, more particularly, to a shortening method which is effected in a simple operation by the vehicle driver.
A method of shortening the braking distance is disclosed in DE 89 11 963 U1 in which a prebraking for a period of about 0.5 s is initiated on the basis of two available signals. The first signal is in this case triggered by a switch being operated by the vehicle driver. It is possible for this switch to be operated by the left foot or by hand. In the case of hand operation, the switch advantageously is arranged such that the hand does not have to be moved away from the steering wheel. The second signal is triggered by the speed at which the vehicle driver removes his foot from the gas pedal being evaluated. If this speed is above a certain threshold value, the prebraking is automatically initiated.
In the known method, disadvantages arise to the extent that a switch has to be additionally operated by the vehicle driver whenever a strong braking operation in the region of a full braking is required. Since very strong braking operations only occur relatively rarely, it may happen that the vehicle driver does not separately operate the switch quickly enough in the event of danger, with the result that a braking operation produced merely by the position of the brake pedal is initiated. Moreover, the two signals available for evaluation still cannot be used to determine an appropriate level for the brake pressure to be set during prebraking. Rather, a standard value for this level must be fixed in advance.
An object of the present invention is to improve the known method of shortening the braking distance in critical driving situations in such a way that the method is brought into effect by as simple an operation as possible by the vehicle driver.
This object has been achieved in accordance with the present invention by using the exceeding of a first threshold value by the actuation speed of the brake pedal as the only criterion for initiating automatic brake operation, and automatically building up a greater brake pressure than that obtained from the brake pedal position immediately after initiating the automatic braking operation.
One advantage of the present invention is that the level of the brake pressure to be adopted can be defined in concrete terms by the fact that the maximum possible braking power occurs. Alternatively the level of the brake pressure to be adopted can be obtained by adding an additional brake pressure p.sub.zus to the brake pressure obtained from the instantaneous position of the brake pedal so that altogether the brake pressure p.sub.add is adopted. In this latter case, this additional brake pressure can advantageously be determined as a function of the brake pedal angular speed and makes it possible to create a time characteristic of the brake pressure which follows the time characteristic of the position of the brake pedal. As a result, the vehicle driver retains the feeling that the braking operation is still predetermined by the intensity of the actuation of the brake pedal.
The method according to the present invention can be applied in a particularly advantageous way in the case of a vehicle equipped with an antilock braking system (ABS) where a braking operation is always controlled to bring it into the region of safe vehicle handling by the ABS predetermining an upper value p.sub.B,limit for the brake pressure p.sub.B,max or P.sub.add to be adopted.
In the method according to the present invention, the actuation speed of the brake pedal is evaluated to establish whether a first threshold value is exceeded, from which it is then deduced whether the initiation of an automatic braking operation according to the invention is or is not required. This evaluation of the actuating speed of the brake pedal makes it possible to build up at a very early time the brake pressure required for carrying out the automatic braking operation, generally with the result that a considerable shortening of the braking distance can be achieved. This first threshold value can be set much lower if the automatic braking operation is implemented on the principle that the brake pressure to be adopted depends on the actuating speed of the brake pedal than if the automatic braking operation is implemented on the principle that the maximum possible brake pressure is built up immediately.
In a considerable number of cases in which, on the basis of the driving situation, a braking operation is required at a relatively high level, i.e. with a relatively great brake pressure, only a relatively slow build-up of the brake pressure takes place because the vehicle driver does not actuate the brake pedal immediately with full force, but depresses the brake pedal fully only with a time delay. Although also in this case the build-up of the brake pressure brought about by the actuating speed of the brake pedal takes place considerably more quickly than in the case of usual braking operations with safety reserve, the brake pressure build-up still takes place more slowly than would actually be possible.
Even if the vehicle driver actuates the brake pedal with full force, the build-up of the full brake pressure is delayed by the time which is required to depress the brake pedal fully, with the result that here too, an accelerated build-up of the brake pressure is possible by evaluation of the actuating speed of the brake pedal in the case of the braking operation described. Consequently, a shortening of the braking distance can be achieved.